Currently, there are three classes of flow meters that utilize ultrasonic transducers, which include transit time ultrasonic flow meters (TTUF), doppler ultrasonic flow meters (DUF), and cross correlation ultrasonic flow meters (CCUF).
CCUF's measure the time required for ultrasonic beams to transit across a flow path at two, axially displaced locations along a pipe. Within this measurement principle, variations in transit time are assumed to correlate with properties that convect with the flow, such as vortical structure, inhomogenities in flow composition, temperature variations to name a few.
CCUF's utilize high frequency acoustic signals, i.e. ultrasonics, to measure much lower frequency, time varying properties of structures in the flow. Like all other cross correlation based flow meters, the physical disturbances which cause the transit time variations should retain some level of coherence over the distance between the two sensors.
Cross correlation ultrasonic flow meters have been around since the early 1960's. CCUF's are typically much more robust to variations in fluid composition than the other ultrasonic-based flow measurement approaches such as transit time and Doppler based methods.
Although CCFU's are operationally more robust than other ultrasonic interpretation techniques, they suffer from drawbacks attributed to most cross correlation flow meters, i.e., they are have slow update rates and relatively inaccurate.
Transit time, defined as the time required for an ultrasonic beam to propagate a given distance, can be measured using a radially aligned ultrasonic transmitter and receiver. For a homogenous fluid with a no transverse velocity components flowing in an infinitely rigid tube, the transit time is given by the following relation:t=D/Amix
t is the transit time, D is the diameter of the pipe, and Amix is the speed of sound propagating through the fluid.
In such a flow, variation in transit time is analogous to a variation in sound speed of the fluid. In real fluids however, there are many mechanisms, which could cause small variations in transit time which remain spatially coherent for several pipe diameters. For single phase flows, variations in the transverse velocity component will cause variations in transit time. Variations in the thermophysical properties of a fluid such as temperature or composition will also cause variations. Many of these effects convect with the flow. Thus influence of transverse velocity of the fluid associated with coherent vortical structures on the transit time enables transit time based measurements to be suitable for cross correlation flow measurement for flows with uniform composition properties. The combination of sensitivity to velocity field perturbation and to composition changes make transit time measurement well suited for both single and multiphase applications.
Despite CCUFs functioning over a wide range of flow composition, standard transit time ultrasonic flow meters (TTUF) are more widely used. TTUF tend to require relatively well behaved fluids (i.e. single phase fluids) and well-defined coupling between the transducer and the fluid itself. TTUFs rely on transmitting and receive ultrasonic signals that have some component of their propagation in line with the flow. While this requirement does not pose a significant issue for in-line, wetted transducer TTUFs, it does pose a challenge for clamp-on devices by introducing the ratio of sound speed in the pipe to the fluid as an important operating parameter. The influence of this parameter leads to reliability and accuracy problems with clamp-on TTUFs.
CCFUs, utilize ultrasonic transducers to launch and detect ultrasonic waves propagating normal to the flow path. Refraction of ultrasonic waves at the pipe/fluid interface is not an issue and the ratio between sound speed of pipe and the fluid does not direct effect operability.
The primary issue with CCFUs is that they are slow and inaccurate. CCFUs rely on cross correlation of two measurements utilizing time domain cross correlation.
A flow meter embodying the present invention utilizes an array of ultrasonic transmitters and receivers to observe a measured property of a flow through a pipe, i.e. transit time and/or amplitude, at respective axial location, combined with array beam-forming techniques to create a new type of sonar-based, ultrasonic flow meter to overcome drawbacks of current ultrasonic meters
Using sonar-based array processing methods to interpret the output of multiple transit time transmitting and receiving sensor pairs would result in enhanced performance over the current CCFUs. The enhanced performance would include better accuracy, faster update rate and more robust operation.